U.S. patent number 3,812,352 [Application Number 05/284,382] was granted by the patent office on 1974-05-21 for encoder readout system.
This patent grant is currently assigned to Itek Corporation. Invention is credited to Alan J. MacGovern.
United States Patent |
3,812,352 |
MacGovern |
May 21, 1974 |
ENCODER READOUT SYSTEM
Abstract
A system for reading out positional information from a linear or
rotary encoder based on an extension of the pinhole imaging theory
to include the effects of diffraction in the encoder. In a typical
encoder, light from a radiation source is directed first through a
code plate having thereon periodic markings of alternating
transparent and opaque increments, then through a slit plate also
having thereon periodic markings of alternating transparent and
opaque increments, and then upon a detector. According to the
teachings of this invention there are optimal positions for the
encoder plate, the slit plate and the detector to obtain maximum
readout signals from the encoder. These optimal positions are
selected in theory according to two relationships which must be
simultaneously satisfied, namely: Z.sub.1 =nS.sub.1 S.sub.2
/.lambda., and Z.sub.1 = (S.sub.1 - S.sub.2)Z.sub.2 / S.sub.2 where
n is an integer of one or greater, .lambda. is the mean wavelength
of the light source, S.sub.1 is the length of one clear
increment-opaque increment period in the code plate, S.sub.2 is the
length of one clear increment-opaque increment period in the slit
plate, Z.sub.1 is the distance between the code plate periodic
markings and the slit plate periodic markings, and Z.sub.2 is the
distance between the detector and the periodic markings on the slit
plate.
Inventors: |
MacGovern; Alan J. (Acton,
MA) |
Assignee: |
Itek Corporation (Lexington,
MA)
|
Family
ID: |
23090001 |
Appl.
No.: |
05/284,382 |
Filed: |
August 28, 1972 |
Current U.S.
Class: |
250/237G;
356/395 |
Current CPC
Class: |
G01D
5/38 (20130101) |
Current International
Class: |
G01D
5/38 (20060101); G01D 5/26 (20060101); G01b
011/04 (); H01j 005/02 () |
Field of
Search: |
;250/237G ;356/169 |
Other References
burch, J. M., "The Possibilities of Moire-Fringe Interferometry,"
Interferometry, A Symposium Held at the National Physical
Laboratory on 9th, 10th, 11th June, 1959, Her Majesty's Stationery
Office, 1960, title page, page 181, 198-200 relied upon..
|
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Grigsby; T. N.
Attorney, Agent or Firm: Blair; Homer O. Nathano; Robert L.
Roch; William C.
Claims
1. In an encoder which measures the movement of a first plate,
having thereon periodic markings of alternating transparent and
opaque increments, relative to a second plate, having thereon
periodic markings of alternating increments, the improvement
comprising the encoder and the readout system in the encoder being
optimally designed while taking into account diffraction effects of
radiation in the encoder and comprising:
a. a first plate having thereon periodic markings of alternating
transparent and opaque increments, with the length of one clear
increment-opaque increment period being S.sub.1 ;
b. a second plate having thereon periodic markings of alternating
increments, with the length of one alternating increment period
being S.sub.2 ;
c. a radiation source means for producing radiation having a mean
wavelength .lambda. and for directing it first through said first
plate and then onto said second plate, with relative movement
between said first plate and said second plate causing modulation
of the radiation;
d. a detector means for detecting modulation of the radiation by
said first and second plates; and
e. means for detecting and reading relative movement between said
first plate and said second plate while taking into account
diffraction effects, and including means for positioning said first
plate and said second plate and said detector means substantially
according to the following two relationships:
Z.sub.1 = nS.sub.1 S.sub.2 1.lambda., and
Z.sub.1 = (S.sub.1 - S.sub.2)Z.sub.2 /S.sub.2
wherein S.sub.1, S.sub.2, and .lambda. have already been defined,
and wherein Z.sub.1 equals the distance between the first plate
periodic markings and the second plate periodic markings, Z.sub.2
equals the distance between said detector means and the periodic
markings on said
2. An encoder as set forth in claim 1 wherein said second plate
has
5. An encoder as set forth in claim 1 wherein the encoder is a
linear
6. An encoder as set forth in claim 1 wherein the encoder is a
rotary
7. An encoder as set forth in claim 1 wherein said first plate is
the
8. An encoder as set forth in claim 2 wherein the encoder is a
linear
9. An encoder as set forth in claim 8 wherein said first plate is
the
10. An encoder as set forth in claim 2 wherein the encoder is a
rotary
11. An encoder as set forth in claim 10 wherein said first plate is
the
12. An encoder as set forth in claim 1 wherein said second plate
has periodic markings of alternating reflective and absorptive
increments.
13. An encoder as set forth in claim 12 wherein the encoder is a
linear
14. An encoder as set forth in claim 13 wherein said first plate is
the
15. An encoder as set forth in claim 12 wherein the encoder is a
rotary
16. An encoder as set forth in claim 15 wherein said first plate is
the encoder code plate and said second plate is the encoder slit
plate.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to analog to digital
encoders for converting an analog rotary or linear movement into a
digital signal representative of that movement. More particularly,
the present invention relates to a new and improved readout system
for such an encoder wherein all of the elements in the readout
system are optimally positioned, in theory, according to two
relationships which must be satisfied.
In the field of analog to digital encoders, it has been the general
practice to position the components of the readout system according
to an equation derived from pinhole imaging theory, namely:
Z.sub.1 = (S.sub.1 -S.sub.2)Z.sub.2 /S.sub.2
wherein Z.sub.1 equals the distance between the encoder track on
the code plate and the encoder track on the slit plate, Z.sub.2
equals the distance between the encoder track on the slit plate and
the detector, S.sub.1 is the length of one clear increment-opaque
increment period on the code plate, and S.sub.2 is the length of
one clear increment-opaque increment period on the slit plate. Such
prior art readout systems have been somewhat unsatisfactory in that
they resulted in encoders wherein the encoder code plate and the
encoder slit plate were placed as close to each other as possible
and typically one to five thousandths of an inch apart. Such a
small gap meant tolerances had to be very tight in the encoder
which resulted in a more expensive product. Also with such a small
gap, any dust or foreign particles in the encoder presented
problems as they might lodge between the two plates and scratch the
encoder tracks. Further, with such a prior art setup much of the
readout signal was lost due to diffraction effects in the
encoder.
The present invention results in an encoder readout station with a
rather large gap between the code and slit plates. There is a type
of readout station in the prior art which also has a rather large
gap between the code and slit plates. This type of prior art
readout station utilizes collimated light and Fresnel imaging, and
operates on a distinctly different principle from the present
invention which works well with either collimated or noncollimated
light sources.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment, a system for reading out
positional information from linear or rotary encoders is disclosed
which is based on an extension of the prior art pinhole imaging
theory to include diffraction effects.
In accordance with a preferred embodiment, a radiation source
having a wavelength .lambda. is arranged to illuminate a first
plate having thereon periodic markings of alternating clear and
opaque increments, with the length of one clear increment-opaque
increment period being S.sub.1. A second plate is positioned behind
the first plate with the second plate having thereon periodic
markings of alternating increments, with the length of one
increment period being S.sub.2. A detector is positioned to detect
radiation modulated by the first and second plates. The first and
second plates and the detector are, in theory, positioned according
to the following two relationships:
Z.sub.1 = nS.sub.1 S.sub.2 /.lambda., and Z.sub.1 = (S.sub.1
-S.sub.2) Z.sub.2 /S.sub.2
wherein S.sub.1, S.sub.2 and .lambda. are as defined above, Z.sub.1
equals the distance between the first plate periodic markings and
the second plate periodic markings, Z.sub.2 equals the distance
between the second plate periodic markings and the detector, and n
equals an integer of one or greater.
While in a typical encoder light is directed first through a code
plate, then through a slit plate, and finally onto the detector,
other embodiments of encoders might be built having light directed
first through the slit plate, then through the code plate and
finally onto a detector. The teachings of this invention are
applicable to all such embodiments, as long as the spacing between
the first and second plates and the detector are substantially as
set forth in the previous paragraph.
While the preferred embodiment is illustrated as an encoder wherein
light from a radiation source is directed through first and second
plates, each having alternating clear increment-opaque increment
cycles, and then onto a photodetector, other embodiments might be
built wherein light from a radiation source is directed through a
transmissive first plate onto a second plate having alternating
reflective and absorptive increments. The teachings of this
invention are applicable to such an embodiment, with the only other
modification being that the detector (positioned according to
Z.sub.2) would be on the opposite side of the second plate.
An encoder built according to the teachings of this invention has
the advantages of allowing much larger gaps and looser tolerances
between the encoder code and slit plates while obtaining better
modulation of the positional signal out of the encoder. The larger
encoder gap means that the encoder is more tolerant to dirt and
dust in the encoder as the foreign particles are less likely to
lodge between the code and slit plates and scratch the encoder
tracks thereon. Also, the wider gap and looser tolerances simplify
assembly, and result in a more reasonably priced encoder. Also,
very significantly, an encoder built according to the teachings of
this invention utilizes the effects of diffraction in the encoder
to its advantage rather than having diffraction degrade the
performance of the encoder. The improved positional signal
modulation allows the reading of finer code tracks than is
presently possible with pinhole imaging systems, and it is
conjectured that this invention may allow the reading of encoder
tracks up to eight times as fine as it is presently possible to
read with spatially incoherent light sources.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the theory of pinhole imaging in an encoder
readout system.
FIG. 2 shows the emanation of interference bands from a slit
plate.
FIG. 3 illustrates a graph of the measured AC and DC signals versus
the slit to code plates gap for a prior art encoder.
FIG. 4 illustrates a graph of signal modulation versus the slit to
code plates gap for a typical prior art encoder.
FIG. 5 illustrates a graph of the measured AC and DC signals versus
the slit to code plates gap for an encoder built according to the
teachings of this invention.
FIG. 6 illustrates a graph of signal modulation versus the slit to
code plates gap for an encoder built according to the teachings of
this invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 illustrates geometrically the prior art theory of pinhole
imaging in an encoder readout system. A light source 10 illuminates
a periodic grating 12 which is the encoder track on the code plate.
Light passing through the periodic grating 12 then passes through a
periodic grating 14 which is the encoder track on the slit plate,
and is then directed back to a detector 16 positioned behind a
bottom slit grating 17. In alternative embodiments of the prior
art, the one detector 16 and bottom slit grating 17 are replaced by
an array of small detectors, the outputs of which are summed.
According to pinhole imaging theory, essentially each aperture in
the slit plate produces a pinhole image of the code plate. At a
certain distance Z.sub. Z.sub. from the slit plate, the images from
all the apertures fall on top of each other giving a magnified
overall image of the code plate. From this geometry the following
equation has been derived and is generally known in the art:
Z.sub.2 = S.sub.2 /(S.sub.1 -S.sub.2) .sup.. Z.sub.1, wherein
S.sub.1 is the length of a clear increment-opaque period on the
code plate, S.sub.2 equals the length of a clear increment-opaque
increment period on the slit plate, Z.sub.1 equals the distance
between the code plate periodic markings and the slit plate
periodic markings, Z.sub.2 equals the distance between the slit
plate periodic markings and the detector, and A is the distance
between the images at the image plane. A detector is placed in the
image plane, and detects the motion of the code plate with respect
to the slit plate. The detector is placed behind a bottom slit
grating having apertures which are small compared to the distance
A. The image of the code plate is magnified at the detector by the
amount A/S.sub.1 or Z.sub.2 /Z.sub.1.
In a typical prior art encoder S.sub.1 and S.sub.2 might differ by
approximately 1 percent such that Z.sub.2 equals 100 Z.sub.1 and
the magnification equals 100. With a code plate to slit plate
spacing of 0.002 inches the detector would be 0.2 inches behind the
slit plate. Based purely on geometry, one would expect that the
spacing could be increased to perhaps 0.01 inches and the detector
moved back to one inch, and that satisfactory results would still
be obtained. In general this has not been born out in practice and
is a false expectation as the simple geometrical treatment by the
prior art does not take into account diffraction.
The present invention is the result of the application of
diffraction theory to pinhole imaging theory. The present invention
results in some surprising conclusions and encoder readout systems
which are superior to prior systems in several respects.
The following explanation is useful to understand the effects of
diffraction in encoder readout systems.
Referring to FIG. 2, consider light reaching two adjacent apertures
a and b in the slit plate from a particular aperture a' in the code
plate. In a manner similar to that of Young's double slit
experiments, interference bands emanate to the right apparently
from a point between the two apertures of the slit plate. The lines
AC and AB represent the zero and minus first order of these
interference bands. There is also a set of interference bands
formed by light from adjacent code plate aperture b', and AC and AB
represent respectively the plus first order and zero order of this
interference pattern. From simple diffraction theory for small
angles, the angle CAB is .lambda./S.sub.2 where .lambda. is the
source mean wavelength and S.sub.2 is the slit plate cycle length.
From geometry it then follows that S.sub.1 /Z.sub.1 =
.lambda./S.sub.2, and therefore Z.sub.1 = S.sub.1 S.sub.2
/.lambda.
In fact this equation can be generalized to
Z.sub.1 = nS.sub.1 S.sub.2 /.lambda. wherein n = 0, 1, 2, 3...
(1)
The results of pinhole imaging theory are still valid, and so
also
Z.sub.1 = (S.sub.1 -S.sub.2)Z.sub.2 /S.sub.2 (2)
thus, this invention results in a realization that there are two
equations for Z.sub.1 which must be simultaneously satisfied to
obtain the theoretically optimal positions for the encoder plate,
the slit plate and the detectors.
The inventor has also derived equation (1) above a mathematical
analysis of the effects of diffraction in encoder readout systems.
The Huyghens-Fresnel diffraction integrel was applied to obtain the
optical transfer function for a generalized object illuminated
incoherently and transmitted by some aperture. The resultant
transfer function agreed precisely with that derived by another
approach by Swing and Rooney, J. O. S. A., May 1968, Vol. 58, No.
5, p. 629. Applying the transfer function to the specific situation
of optical encoders leads to equations (1) and (2) as being
appropriate for optimal modulation of the output.
It is interesting to note that when the integer n = 0 is placed in
equation (1), the approach results in the same approach being
persued by the prior art wherein the spacing between the code and
slit plates is kept as small as possible. With a very narrow gap
(with n = 0) the effects of diffraction are not significant.
Using equation (1) for a typical encoder situation in which S.sub.1
= 25 microns, S.sub.2 = 0.99S.sub.1, .about. S.sub.1, .lambda. = 1
micron, and for an integer n = 1, Z.sub.1 .about. S.sub.1.sup.2
/.lambda. = 625 microns or about 0.025 inches. The distance Z.sub.2
to the image plane would then be 2.5 inches, which is somewhat
larger than would be convenient. The distance Z.sub.2 might be
reduced by decreasing the ratio of S.sub.2 to S.sub.1 which would
also result in a consequent reduction in magnification. Reduction
in magnification might cause the fringes in the image plane to be
sufficiently close together to require the use of a bottom slit
grating in the image plane.
FIGS. 3 and 4 illustrate graphs of measurements of an encoder
readout station built according to the conventional pinhole imaging
theory, and FIGS. 5 and 6 illustrate graphs of measurements of the
same encoder readout station positioned according to the teachings
of this invention, with the integer n equal to 1. The readout
station utilized in these measurements had a light source
consisting of a gallium arsenide light emitting diode having a mean
wavelength .lambda. of about 0.93 microns. S.sub.1 was 24.9
microns, S.sub.2 was 22.2 microns, and the utilized detectors were
phototransistors.
FIG. 3 illustrates a graph of that readout station while utilizing
just pinhole imaging theory. The graph is of the detector AC and DC
currents in microamps versus the distance S.sub.1 between the code
and slit plates. In an encoder, one would like to maximize the AC
signal, which gives useful positional information, and minimize the
DC signal, which is essentially noise. As shown by FIG. 3, the AC
current was maximum and the DC current a minimum at the smallest
distance between the code and slit plates. FIG. 4 illustrates the
modulation of current for the same readout station, which
modulation is defined as the AC current over the AC & 2DC
currents. In an encoder one would like to maximize the signal
modulation, and as expected and shown by FIG. 4, the best results
were obtained with the minimum gap between the code and slit
plates.
Typically, one would like a signal modulation of at least 0.5 and a
gap between the code and slit plates of no less than two
thousandths of an inch. Accordingly, with an encoder readout
station having the characteristics set forth in FIG. 4, the code to
slit plate gap would have to be maintained between 2 and 3.2
thousandths of an inch.
FIGS. 5 and 6 illustrate graphs of measurements of the same encoder
readout station positioned according to the teachings of this
invention. Utilizing equations (1) and (2) above and with the
integer n=1, one would expect optimum results at a slit to code
plate gap of 0.023 inches. As shown by FIG. 5, the maximum AC
signal and the minimum DC signal was obtained at just slightly
greater than 0.019 inches. The slight discrepancy between the
theoretically expected optimum gap and the gap at which the best
signals were obtained may be due to any of the following. The mean
wavelength of the light source might not have been at the
anticipated wavelength of 0.93 microns. The code and slit plates
were for a rotary encoder wherein the encoder tracks have a
configuration with the increment periods varying from the outer
radius to the inner radius, while the theory was worked out for a
linear configuration wherein the increment periods are fixed. Also,
it is possible that the mathematical derivation referred to above
might not have been rigorous enough, and possibly a more rigorous
derivation might result in several extra terms having a slight
effect on the anticipated optimal gap for Z.sub.1. Also, it is
possible that the discrepancy may have been caused by phase
corrugations in the emulsion which were not accounted for in the
derivation. In any event, the measured results are substantially
the same as predicted by equations 1 and 2 above. Perhaps the best
way to design an encoder while utilizing the teachings of this
invention would be to position the code to slit plate gap according
to the theoretically expected position, then empirically measure
the output of the encoder at slightly different gaps, and then
select the gap giving the optimum results.
Referring back to FIG. 5 and 6 and comparing the results shown
therein with the results shown in FIGS. 3 and 4, the following
significant differences can be seen.
The first significant difference is that a readout station
constructed in accordance with the prior art would have a mean code
plate to slit plate gap of approximately 0.0026 inches, whereas a
readout station constructed according to the teachings of this
invention would have a mean gap around 0.019 inches. With a mean
gap of 0.0026 inches, excessive play between the two plates might
have dire results as any contact between the plates might scratch
the encoder tracks located thereon. Also, any foreign particles in
the encoder might lodge between the plates, and result in scratched
encoder tracks. This difficulty is obviously removed when the mean
encoder gap may be maintained at 0.019 inches.
A second significant difference is the range through which the slit
to code plate gap might vary. As mentioned before, the prior art
readout station would probably be designed to operate between 0.002
and 0.0032 inches. Referring to FIG. 6 and the characteristics of a
readout station constructed according to this invention, it may be
seen that a signal modulation of 0.5 is present over the range of
0.0162 to 0.0228 inches. This presents a range of 0.0062 inches
through which the encoder gap might vary. When comparing this with
0.0012 inches of the prior art, one sees that the range is expanded
by several hundred percent with the present invention, which is
indeed a significant improvement.
A third evident advantage of a readout station constructed
according to this invention is that the encoder gap might be
designed to be very close to 0.019 inches wherein the signal
modulation is above 0.9. In the prior art a signal modulation of
this magnitude is almost impossible to obtain as the encoder gap
would have to be held around 0.001 inches. The very low tolerances
which would be required to operate with such an encoder gap would
make the cost of the encoder extremely expensive. Thus, an encoder
built according to the teachings of this invention results in
better signal modulation, while allowing the encoder to be built to
less stringent tolerances. The result is a better encoder at less
cost.
The above example illustrates how the teachings of this invention
may be utilized to build an encoder which requires substantially
less stringent tolerances than an encoder built according to the
prior art. On the other hand, if tolerances of the prior art were
adhered to, the teachings of this invention might be utilized to
build an encoder capable of reading out much finer positional
information than the prior art. For example, in the equation above,
Z.sub.1 = n S.sub.1 S.sub.2 /.lambda.. In most instances S.sub.1
.congruent. S.sub.2, so that the equation may be generalized to
S.sub.1 = nS.sub.1.sup.2 /.lambda.. If S.sub.1 were made twice as
fine (i.e., S.sub.1 /2), S.sub.1.sup.2 in the above equation would
cause Z.sub.1 to be reduced by one fourth. Thus, in the above
example if the code track S.sub.1 were made twice as fine, the
calculated value for Z.sub.1 would be 0.00575 inches. The inventor
has in fact carried out such an experiment, and measurements of the
encoder output indicate that the best results are obtained at
0.00475 inches, which is substantially the value predicted by the
above equations.
The preferred emodiment is illustrated as a transmissive encoder
wherein light from a radiation source is directed through first and
second plates, each having alternating clear increment-opaque
increment cycles, and then onto a photodetector. In another
embodiment, the teachings of this invention may be applied to an
encoder wherein light from a radiation source is directed through a
transmissive first plate onto a second plate having alternating
reflective and absorptive increments. The radiation would then be
selectively reflected by the second plate onto a detector. In such
an embodiment, the detector (positioned according to Z.sub.2) would
be on the opposite side of the second plate.
While several embodiments have been described, the teachings of
this invention will suggest many other embodiments to those skilled
in the art.
* * * * *